1. Field of the Invention
The present invention relates to occupancy and vacancy sensors for detecting an occupancy or a vacancy in a space, and more particularly, to an ultrasonic receiving circuit for an occupancy or vacancy sensor.
2. Description of the Related Art
Occupancy and vacancy sensors are often used to detect occupancy and/or vacancy conditions in a space in order to control an electrical load, such as, for example, a lighting load. An occupancy sensor typically operates to turn on the lighting load when the occupancy sensor detects the presence of a user in the space (i.e., an occupancy event) and then to turn off the lighting load when the occupancy sensor detects that the user has left the space (i.e., a vacancy event). A vacancy sensor only operates to turn off the lighting load when the vacancy sensor detects a vacancy in the space. Therefore, when using a vacancy sensor, the lighting load must be turned on manually (e.g., in response to a manual actuation of a control actuator).
Occupancy and vacancy sensors have often been provided in wall-mounted load control devices that are coupled between an alternating-current (AC) power source and an electrical load for control of the amount of power delivered to the electrical load. Some prior art occupancy and vacancy sensors have been provided as part of lighting control systems. These sensors are often coupled via a wired control link to a lighting controller (e.g., a central processor), which then controls the lighting loads accordingly. Alternatively, the sensors may be battery-powered and may be operable to transmit wireless signals, such as radio-frequency (RF) signals, to load control devices, such as dimmer switches. These occupancy and vacancy sensors are not required to be mounted in electrical wallboxes, but may be mounted to the ceiling or high on a wall. Therefore, the occupancy and vacancy sensors may be positioned optimally to detect the presence of the user in all areas of the space.
The prior art occupancy and vacancy sensors typically comprise internal detectors, such as, for example, a pyroelectric infrared (PIR) detector, and a lens for directing energy to the PIR detector for detecting the presence of the user in the space. Alternatively, some prior art occupancy and vacancy sensors have included ultrasonic transmitting and receiving circuits for detecting the presence of the user in the space. Ultrasonic sensors transmit ultrasonic waves at a predetermined frequency, and analyze received ultrasonic waves to determine if there is an occupant in the space. The received ultrasonic waves that are reflected off of moving objects will be characterized by a Doppler shift with respect to the transmitted ultrasonic waves, while the received ultrasonic waves that are produced by reflections off of the walls, ceiling, floor, and other stationary objects of the room will not have a Doppler shift. Therefore, ultrasonic occupancy and vacancy sensors are able to determine if there is an occupant in the space if there is a Doppler shift between the frequencies of the transmitted and received ultrasonic waves.
Generally, the size of the objects that produce the ultrasonic waves having the Doppler shift (such as, a moving hand) are very small, and thus produce reflected ultrasonic waves having small magnitudes. One of the issues with detecting ultrasonic waves having a Doppler shift is that these received ultrasonic waves can be difficult to distinguish from the received ultrasonic waves that do not have a Doppler shift. A figure of merit for occupancy detection limits can be described using the signal-to-interference ratio (SIR), which is the ratio of the Doppler-shifted ultrasonic waves expressed in sound pressure level (SPL) to the non-Doppler-shifted ultrasonic waves.
The standard implementations for detecting Doppler shifts in ultrasonic waves take three standard forms. The first form uses a phase-lock-loop (PLL) integrated circuit (IC), such as, for example, part number CD74HC7046, manufactured by Texas Instruments Incorporated. In this implementation, the received ultrasonic waves are amplified by a pre-amplifier and then compared with a single fixed threshold (e.g., 100 mV) using a comparator to yield a binary waveform. The binary waveform is then applied to an exclusive-or (XOR) gate where the second input to the XOR is a clock input (e.g., a 40-kHz clock signal) that also drives the ultrasonic transmitting circuit. The resulting signal is then passed through a band-pass filter to extract the Doppler signal. The resulting Doppler signal is then compared to a fixed threshold using another comparator to detect an occupancy or vacancy condition. The drawback of this implementation is that the circuit is very sensitive to the thresholds of the comparators and only works on signals with an SIR greater than approximately −40 dB.
The second prior art form for detecting Doppler shift implements the detection algorithm primarily within a microcontroller. In this implementation, the received ultrasonic waves are amplified by a preamplifier and then sampled using an analog-to-digital converter (e.g., an 8 to 12-bit ADC) in the microcontroller. The remainder of the algorithm is essentially the same as in the first prior art form for detecting Doppler shift described above except that the remainder of the algorithm of the second form is executed in the software of the microcontroller. This implementation depends on the accuracy of the ADC of the microcontroller and is limited by numerical noise due to the ADC quantization and the numerical precision used to calculate the results, which thus limits the ability to detect small-magnitude ultrasonic waves that have a Doppler shift.
The third prior art form is an amplitude-modulation (AM) demodulator, which, in its simplest form, uses a diode and a low-pass filter to form an envelope detector. The limitation of this circuit is that the received ultrasonic signal must have a minimum amplitude to render the diode conductive, thus reducing the ability of the circuit to detect small-magnitude ultrasonic waves that have a Doppler shift.
Thus, there is a need for a simple, effective low-cost ultrasonic receiving circuit for use in occupancy or vacancy sensors that avoids the disadvantages of the prior art.